(1) Field of the Invention
The present invention relates to a high power factor power-supply system. More particularly, it relates to a resonant converter which achieves stable output control and improves input power factor over large fluctuations in input voltages and loads.
(2) Description of the Prior Art
Generally, a converter that changes an alternating current power source into a direct current power source is used to supply power at a predetermined level to a load. As shown in FIG. 1a, an alternating current power source Vs is rectified by a full-wave rectifier circuit, a smoothing capacitor (C) connected to the full-wave rectifier circuit, and a DC--DC converter (DC to DC converter), to supply direct current power to load R.sub.L. As shown in FIG. 1B, the phase relationship of input current i and voltage V.sub.s is modified by the circuit shown in FIG. 1A. The modification results from the influence of the capacitor C that deteriorates the power factor. The equation for power factor is as follows, wherein voltage and current should be equal to each other and in phase: ##EQU1## wherein W is a consuming power, and V and A are virtual values of the voltage and current at the alternating current input's side.
FIG. 2A shows a power supply system circuit improving power factor, which has been made by the present inventor. As shown in FIG. 2B, the power factor is improved to a large degree.
The circuit improving power factor is similar to the circuit shown in FIG. 1A. In this circuit a switching device Q is turned on or off at suitable times, so that inductor L transmits a rectified wave form.
A control circuit turning on or off switching device Q uses comparator WC to compare a rectified signal derived from input power Vs with a rectified signal obtained after the input power (Vs) passes through a transformer T1. The resultant value is amplified by a current amplifier IE to generate a switching control signal.
In the circuit of FIG. 2A, current increases to a predetermined current maximum when blocking diode D is turned off, and current decreases to a current minimum when switching device Q is turned off and blocking diode D is turned on. The input current i is limited to having a predetermined width, and the virtual value of the current is controlled to improve power factor.
The circuit of FIG. 2A uses the principle of a boosting type converter. Such systems are inherently complicated, and the transformed current is large. The weight of such systems are heavy and the reliability is decreased because of poor switching characteristics. As shown in FIG. 2B, the conventional circuit has the further disadvantages that there are a dead angle of current and a dead time of current.